Making mounds out of molehills? The role of pocket gophers in the Mima mounds and vernal pools of the San Joaquin Valley (Part One)

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By Dr. Sarah Reed, UC Berkeley. (Dr. Reed recently received her PhD, based in part on mima mounds studies she carried out in the Reserve and on nearby ranchlands)

Mound and trough are Siamese twins, joined by an invisible, crucial stratum that holds the ponded water. -- Hans Jenny

What are Mima mounds?
One of the most visually stunning, and scientifically puzzling, geologic features of North America are the small rounded hillocks called Mima mounds. Mima mounds are circular soil mounds found in grassland landscapes in nearly all states west of the Mississippi, as well as many places around the globe including South America and Africa.

A single mound is small, ranging from 1 to 50 meters (m) in diameter and 0.1 to 2 m in height1. When observing the mounds from a landscape-scale perspective, however, their density and regularity are notable. Named after mounds on Washington’s Mima prairie, Mima mounds cover large expanses of North America and can number more than 40 mounds per hectare2. In many locations, including the Merced area, the depressions adjacent to the mounds seasonally fill with water, creating vernal pools.

Figure 1. Dense Mima mounds on the ~3 million year old Laguna formation. Photo is taken looking south from the Chance Ranch adjacent to the UC Merced campus.

How did the mounds and pools form?

And how do the relatively small features persist given erosion by rain, overland flow, grazing and human activity? In spite of the global distribution, important biodiversity, and visual intrigue of Mima mounds, controversy over their origin has been ongoing for nearly two centuries1, 3. As Higgins4 wrote, “they are a mystery that has been discussed for over 150 years and ... have generated a greater variety of hypotheses than any other geologic feature.” More than 30 different ideas have been proposed, and most of these fall into five general categories: erosional, depositional, seismic, biologic, and shrink-swell.

Have you seen Caddyshack?

While the range of explanations is diverse, in the last several decades of research, the majority of evidence suggests that biologic forces play a key role. For my dissertation (based out of UC Berkeley with advisor Ron Amundson), I explored the hypothesis that attributes the mounds to the work of burrowing rodents, in particular, gophers. Could these pesky varmints really be responsible for millions of acres of undulating topography around the globe?

According to the biologic hypothesis, burrowing animals build Mima mounds in regions with thin, seasonally saturated soils. Given the higher risks of flooding and predation in such environments, the animals seek out deeper or better-drained locations. As they forage and burrow, which involves dislodging and moving soil in order to create an extensive tunnel system, the animals move a collection of soil backwards, toward the center of their activity. Over time, this translocation results in a local accumulation of earth, which serves as preferred habitat for subsequent generations of burrowers 5.

The organism most often implicated in this mound building is the pocket gopher. The range of pocket gophers runs from Alberta, Canada to Panama and largely overlaps the historical distribution of Mima mounds. In 1923, Grinnell6 declared the pocket gopher California’s most widespread and abundant burrowing rodent and estimated their population to be 1 billion.

Figure 2. Video showing a pocket gopher, an animal which spends nearly all of its life underground and which uses its teeth and claws to build a system of tunnels to live and forage. Video courtesy of Robert Meyer.

Pocket gophers are strikingly diverse in morphology and genetics7, 8. The fossorial mammals spend nearly 100% of their lives underground and have adapted to the conditions of the subterranean niche: negligible light, excess moisture, relatively constant temperatures, and low oxygen levels9. As a result, gophers provide an excellent opportunity to explore biologic response to changing soil conditions. Given the intimate interplay between pocket gophers and their earthen habitat and given the proposal that biologic response to particular soil conditions has led to the widespread Mima mound features, my thesis work focused on an age gradient of soils within and around the UC Merced campus, to test gopher response to changing soil conditions.

Figure 3. Typical tailings piles from pocket gopher (Thomomys bottae) tunneling. An average gopher tailings pile is ~25 centimeters (cm) in diameter. A variety of ages of tailings can be seen, ranging from older piles of sediment, which are hardened over by past rain events and/or overgrown with vegetation to new tailings piles, composed of loose soil and no overlying vegetation.

What is the evidence for the ‘gopher hypothesis’?

Many decades of research3 have provided evidence supporting a biologic hypothesis, including:

Mima mounds are all found in regions with thin soil layers over a dense, impermeable substratum

Mounds occur only within geographic range of gophers

Occupancy (or evidence of past occupancy) by gophers of most Mima mounds

Uphill and moundward (towards the mound center) movement of soil by gophers (based on tracer studies of Cox and collaborators10)

Mounds have soils of a size small enough to be transported by gophers

Larger particle sizes at base and in between mounds.

However, much of the evidence is circumstantial, observed only in small regions over short periods of time, and points to inhabitation of mounds by burrowing animals (using preexisting topography as preferred habitat), but does not directly prove that they are the builders of the mounds. In addition, the exact mechanisms of how mounds might be built by burrowing animals and how changing environmental conditions might affect such ecosystems is underexplored. (More on mima mounds next month when Sarah will provide Part 2!)